An ion source is a device that ionizes gas molecules and then focuses, accelerates, and emits them as a narrow beam. This beam is then used for various technical and technological purposes such as cleaning, activation, polishing, thin-film coating, or etching. An ion source is also used for overcoating of various objects by sputtering. Sputtering is a phenomenon that occur when energetic ionized particles, emitted, e.g., from an ion source, impinge on the surface of a solid or liquid target, causing the emission of particles and erosion of the surface of the solid. The sputtered target particles can appear as charged or neutral atoms or molecules, atom clusters, or chunks. The controlled deposition of sputtered particles to form thin films and coatings has industrial application in electronics, optics, and corrosion and wear-resistant coatings.
In a simplest form an ion-beam sputtering system consists of a sealed vacuum chamber that contains an ion-beam source, a target of a sputterable material, and an object to be treated. The target is installed at angle to the direction of the ion beam emitted by the ion source so that at the point of collision with the surface of the target the beam knocks-out or sputters particles of the target material which fly away from the surface of the target. A part of the sputtered particles deposited onto the surface of the treated object. Such a system is described by Brian Chapman in: Glow Discharge Processes (Sputtering and Plasma Etching), John Willey & Sons, New York, 1980, p. 272).
A disadvantage of the ion-beam sputtering system of the type described above is that a significant part of the particles is scattered away from the object and contaminates the inner walls of the chamber and the surfaces of the ion-beam source.
Attempts have been made to reduce scattering and loss of the sputtered particles in the ion-beam sputtering systems described in U.S. Pat. No. 6,130,507 issued on Oct. 10, 2000 to Yu. Maishev et al. now U.S. Pat. No. 6,130,507 and entitled "Cold-Cathode Ion Source with Propagation of Ions in the Electron Drift Plane".
Since the ion source is an essential part of the system, given below is a detail description of a closed-loop ion source 10 of the invention which is shown in a cross-sectional view in FIG. 1. This source is used for emitting ion beams in a radial outward direction in a plane of drift of electrons. In a transverse cross-section (not shown), ion source 10, as well as its appropriate parts such as a cathode, anode, and magnet, may have a circular, oval, or elliptical cross section. It is understood that, strictly speaking, oval or ellipse do not have a radial direction and that the word "radial" is applicable to a circle only. However, for the sake of convenience, here and hereinafter, including patent claims, the terms "radially inwardly" and "radially outwardly" will be used in connection with any closed-loop configuration of the ion-emitting slit from which the ion beams are emitted inwardly or outwardly perpendicular to the circumference of the ion-beam housing.
Ion source 10 has a hollow housing 40 made of a magnetoconductive material which is used as a cathode. Housing 40 has a closed flat bottom 44 and a flat top side 46 with a through closed-loop ion-emitting slit 52 formed in the side wall of housing 40 around its entire periphery, approximately in the middle of the height of the source housing.
A working gas supply hole 53 is also formed in the side wall of housing 40.
Hollow housing or cathode 40 contains a similarly-shaped concentric anode 54 which is fixed inside the housing by means of appropriately shaped bodies 56 and 58 of a nonmagnetic dielectric material, such as ceramic. Anode 54 is spaced from the inner walls of cathode 40 at a radial distance G required to form an ionization space 60. In the direction of the height of housing 40, anode 54 is aligned with the position of closed-loop slit 52.
A magnetic-field generation means, which in this embodiment is shown as a permanent magnet 62, is located inside anode 54 and is spaced from the inner surface of the anode. As shown in FIG. 1, magnet 62 is concentric to anode 54 and housing 40 and also has an oval-shaped configuration. It is understood that upper and lower parts 46 and 44 as well as adjacent parts of housing 40, which form ion-emitting slit 52, should be electrically connected. This is achieved by making magnet 62 of a conductive material, e.g., such as SmCo alloy. Alternatively, when an electromagnet is used, these parts may be connected via conductors (not shown).
Anode 54 is electrically connected to a positive pole 64a of an electric power supply unit 64 by a conductor line 66 which passes into housing 40 via a conventional electric feedthrough 68. Cathode 40 is electrically connected to a negative pole 64b of power supply unit 64.
In operation (FIG. 1), vacuum chamber or object OB (not shown) is evacuated, and a working gas is fed into the interior of housing 40 of ion source 10 via inlet opening 53. A magnetic field is generated by permanent magnet 62 in ionization gap G between anode 54 and cathode 40, whereby electrons begin to drift in a closed path within the crossed electrical and magnetic fields. In the case of the device of the invention, the electrons begin to drift in gap G between anode 54 and cathode 40 and in ion-emitting slit 52 in the same plane in which the ions are emitted from the slit.
A plasma 70 is formed between anode 54 and cathode 40 and partially inside ion-emitting slit 52. When the working gas is passed through ionization and acceleration gap G, ion beam IB, which propagates outwardly in the direction shown by arrows C, is formed in the area of ion-emitting slit 52 and in accelerating gap G between anode 54 and cathode 40.
The above description of the electron drift is simplified to ease understanding of the principle of the invention. In reality, the phenomenon of generation of ions in the ion source with a closed-loop drift of electrons in crossed electric and magnetic fields is of a more complicated nature and consists in the following.
When, at starting the ion source, a voltage between anode 54 and cathode 40 reaches a predetermined level, a gas discharge occurs in anode-cathode gap G. As a result, the electrons, which have been generated as a result of ionization, begin to migrate towards anode 54. After being accelerated by the electric field, the ions pass through ion-emitting slit 52 and are emitted from the ion source. Inside the ion-emitting slit, the crossed electric and magnetic fields force the electrons to move along closed cycloid trajectories. This phenomenon is known as "magnetization" of electrons. The magnetized electrons remain drifting in a closed space between two parts of the cathode, i.e., between those facing parts of cathode 40 which form ion-emitting slit 52. The radius of the cycloid is, in fact, the so-called doubled Larmor radius R.sub.L which is represented by the following formula: EQU R.sub.L =mcV/.vertline.e.vertline.B,
where m is a mass of the electron, B is the strength of the magnetic field inside the slit, V is a velocity of the electrons in the direction perpendicular to the direction of the magnetic field, and .vertline.e.vertline. is the charge of the electron.
It is required that the height of the electron drifting space in the ion-emission direction be much greater than the aforementioned Larmor radius. This means that a part of the ionization area penetrates into ion-emitting slit 52 where electrons can be maintained in a drifting state over a long period of time. In other words, a spatial charge of high density is formed in ion-emitting slit 52.
When a working medium, such as argon which has neutral molecules, is injected into the slit, the molecules are ionized by the electrons present in this slit and are accelerated by the electric field. As a result, the ions are emitted from the slit towards the object. Since the spatial charge has high density, an ion beam of high density is formed. This beam can be converged or diverged by known technique for specific applications.
FIG. 2 illustrates an example of a sputtering system which consists of ion-beam source 10 and a target holder 100 with a target 110. Target holder 100 is rigidly supported by housing 40 of ion source 10. Target holder 100 is made in the form of a plate 102 attached to housing 40, e.g., by bolts 104, 106, with a funnel-shaped peripheral portion 108 which has an upwardly directed larger diameter portion. The inner taper surface of target holder 100 supports a target 110 which has a shape of a truncated cone. The target is attached to peripheral portion 108 of holder 100, e.g., by gluing or by bolts (not shown), and is made of a material, such as cobalt, which has to be deposited onto an object OB.sub.1 by sputtering.
Since ion beam IB.sub.1 is emitted from a closed-loop emitting slit 52 of ion source 10 in a radial outward direction, continuously over the entire periphery of the ion source, and since the plane of target 110 is inclined to the direction of incident beam IB.sub.1 (the angle of attack of the ion beam should be different from 90.degree.), the beam sputters particles of the target, in accordance with conventional sputtering technique, and deposits them onto the surface of object OB.sub.1 in the form of a converging or diverging beam of sputtered particles. The convergence or divergence of the sputtering beam depends on the taper angle of the target and the position of the object with respect to the ion source.
As shown in FIG. 2 sputtering beam PB1 covers the entire surface of object OB.sub.1 so that this surface can be coated with a thin uniform layer of the target material.
In order to improve uniformity of deposition of sputtered particles onto object OB.sub.1, the target 110 can be attached to a moveable target holder (not shown) that performs rotating or swinging motions, or the ion beam can be scanned over the surface of the target. Such mechanisms are shown in the aforementioned early patent application.
It is understood that the entire source-target system and an object to be treated are placed into a sealed vacuum chamber (not shown).
Although the use of plate-like conical targets of the type shown in FIG. 2, as well as the use of movable targets and target scanning ion beams, can to some extent control and reduce scattering of sputtered particles, they cannot prevent scattering completely.
Thus, a common disadvantage of the aforementioned sputtering systems, as well as of any other known sputtering system, is that they do not exclude deposition of at least a fraction of sputtered particles onto surfaces other than the object, e.g., onto the inner walls of the vacuum chamber, surfaces of the ion-beam source, etc. In other words, the known sputtering systems cannot provide directed deposition of sputtered particles essentially only onto the object or prevent some scattering of the deposition material away from the object. The known ion-beam sputtering systems of the aforementioned type do not allow formation of a neutral beam that may be effectively used for overcoating in processes where treatment with ion beams is undesirable. They do not allow adjustment in a ratio of ions to atoms of the deposition material that reach the surface of the object.